It is known to subject sewage which is loaded with organic substances to biological purification, with the organic substances contained in the sewage being transformed microbically. In this microbic transformation, the substances contained in the sewage are converted into harmless metabolic products and other microbic substances, biological sludge being produced thereby. The biological sludge may be separated either by separation in sedimentation or flotation plants based on the different densities, or by filtration in a membrane filter or ultrafilter so that a virtually total solid-liquid separation is achieved, to produce an effluent with a very high degree of purity.
A method of the latter type, in which the microbic transformation takes place under superatmospheric pressure and the separation of the treated sewage from the biological sludge is effected by filtration in a membrane filter or ultrafilter, is disclosed in DE 37 09 174 C2. This document details microbic transformation under superatmospheric and separation of the treated sewage from the biological sludge by membrane filtration or ultrafiltration.
The method known from DE 37 09 174 C2 has been extremely successful in purifying sewage loaded with biologically decomposable organic substances. In this method, in which transformation takes place under superatmospheric pressure, increased biomass concentrations can be obtained by the almost total solid-liquid separation which is largely independent of the physical properties of the micro-organisms. The to biomass concentrations improve performance by several times over that of earlier methods. The total solid-liquid separation makes it possible to effect extremely high degrees of purity in the effluent, up to the retention of all bacteria. The pressures at transformation and filtration can largely be set uniformly, so that the active biomass will not be exposed to excessive differential pressures that might be detrimental.
Organic substances dissolved in the sewage, which are difficult to decompose biologically or not decomposable biologically, as well as organic and inorganic substances whose particles are of a smaller size than is separated by the membrane, are still contained in the treated sewage which has been separated from the biological sludge, thus loading the natural waters which receive the treated effluent, or limiting any further industrial utilization of the treated sewage.